The present invention relates to an electronic key telephone set in an electronic key telephone system, and more particularly, to a key dial circuit.
Known key telephone systems of conventional design have at least one pair of conductors for each line running from the key service unit (KSU) to each station set, and accomplish switching of any selected line at the station set by means of a key or pushbutton associated with the line. Consequently, the number of conductor pairs increases as the number of central office line increases, with accompanying increases in cost of materials and labor required to effect an installation. To overcome these problems, electronic key telephone systems have been designed to effect switching at the KSU instead of in the station set, using reduced-pair cable runs to establish voice signal and data signal and power transmission paths between the KSU and each station set. In such systems, the actuation of a key or pushbutton at a station set causes transmission of data from the station set to the KSU, thereby signalling the desired service, e.g., connection to a central office (C.O.) line. The KSU might employ time division or space division techniques, or both, with relays and semiconductors being employed to implement space division.
In order to send out a selection signal, for example, a multi-frequency signal (hereinafter called "DTMF signal"), a multi-frequency signal generator circuit for generating a DTMF signal is provided in every electronic key telephone set in a known electronic key telephone system. The provision of the DTMF signal generator circuit in every key telephone set makes not only the key telephone set but also the entire key telephone system expensive.
In another known system proposed in Japanese Patent Application No. 104921/1979, the DTMF signal generator is removed from an electronic key telephone set, instead provision is made such that a digital signal corresponding to a depressed key in a key dial may be transmitted from the key telephone set onto data lines for the KSU and the DTMF signal may be sent commonly from a sender in the KSU to the C.O. line, to thereby achieve economization.
In order to maintain telephone service during power failures, a known key telephone system has additional relays in the KSU for connecting each C.O. line to a predetermined station set. However, since the key telephone set in the latter system lacks a selection signal generator circuit, any key telephone set would be unable to originate a call during power failure, and therefore, the key telephone system would become useless.